Sudanese planes have reportedly bombed villages in remote areas of western Sudan, causing thousands of refugees to flee across the border into Chad.
Refugees have said that villages in the Darfur province are attacked daily.
Arab militiamen, backed by Sudanese Government forces, have been fighting a year-long rebel uprising in the area.
The United Nations refugee agency (UNHCR) estimates that more than 100,000 people have fled the province since fighting began.
'Difficult to find'
The agency also says it is concerned about the harsh border terrain and poor weather conditions refugees are forced to endure - with scorching sun during the day and bitter cold at night.
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The planes just want to destroy places with civilians - the rebels are elsewhere
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"You can pass within... metres of where people are under a tree and you might not even notice them because of the terrain," UNHCR spokeswoman Kitty McKinsey told the BBC.
"It's not like previous refugee movements from Rwanda or from Kosovo in 1999, for example, where you suddenly had hundreds of thousands of people all together.
"These people are quite dispersed and a bit difficult to find."
The fighting in Darfur has worsened as peace talks to end the war in the south of the country have progressed.
Those who have been forced to flee their homes accuse the government of using aircraft to bomb their villages, with militia groups then mounting follow-up raids.
BBC correspondent Grant Ferrett says many of those crossing the border have walked for several days, having left their homes with few, if any, possessions.
'Always bombing'
One man, a schoolteacher, said his grandmother was killed when a bomb crashed through his home. His managed to pull his teenage brother free from the debris but he died later from his injuries.
"It is terrible, they are slaughtering us," Ishmael Haggar told the Associated Press news agency.
"I need to tell somebody."
Medical workers told Reuters news agency that the number of refugees crossing the border with severe injuries sustained from shrapnel has increased in the past few weeks and more are arriving every day.
"They're always bombing," injured refugee Abakar Muktar told Reuters.
"The planes just want to destroy places with civilians. The rebels are elsewhere."